The present invention relates to a hybrid drive according to the characteristics of Claim 1.
In the case of many hybrid concepts known from the state of the art, the crankshaft of an internal-combustion engine is connected by way of a first clutch with the rotor of an electric machine and by way of a second clutch with the input shaft of a stepped automatic transmission. In the “parallel operating mode”, i.e. when the vehicle is driven simultaneously by the internal-combustion engine as well as by the electric machine, the internal-combustion engine and the electric machine thereby rotate at the same rotational speed or, if they are coupled with one another by way of a “transmission ratio”, corresponding to a predefined rotational-speed ratio, which is not always optimal with respect to power.
It is an object of the invention to create a novel hybrid drive concept which better takes into account the above-described disadvantages of conventional hybrid drive concepts.
This object is achieved by means of the characteristics of Claim 1. Advantageous embodiments and further developments of the invention are contained in the subclaims.
The starting point of the invention is a hybrid drive having a first driving machine, which may be formed by an electric machine, and a second driving machine, which may be formed by an internal-combustion engine. The two driving machines are mutually coupled by way of a “torque superposition device”. The term “torque superposition device” should be interpreted very broadly. The general meaning is that the driving power of the two driving machines or the corresponding drive torques can be “interconnected”, i.e. can be superposed, for example, by means of a summation transmission. Correspondingly, the torque superposition device has a first torque input, which is assigned to the first driving machine, and a second torque input, which is assigned to the second driving machine, as well as a torque output, by way of which the “interconnected” torque can be transmitted to driving wheels of a vehicle.
In the following, the term “electric machine” or “e-motor” will be used for the first driving machine, and the term “internal-combustion engine” will be used for the second driving machine, the above not being understood to be limiting.
The essence of the invention consists of the fact that the electric machine is coupled or can be coupled by way of the first torque input with a “first transmission device”, and the internal-combustion engine is coupled or can be coupled by way of the second torque input with a “second transmission device” of the torque superposition device, and the transmission devices are each coupled, i.e. kinematically connected, on the output side with the torque output. At least one, preferably both transmission devices have at least two alternatively selectable transmissions or transmission positions. Basically, various constructions can be used for the transmission devices, such as shiftable spur gearwheel positions, shiftable planetary transmissions, shiftable belt or chain drives, etc., or also infinitely variable transmission devices.
By means of the hybrid drive according to the invention, the two “power branches” can be shifted independently of one another. Within the scope of the available gears, the rotational speeds of the internal-combustion engine and of the electric machine can be selected independently of one another. During the shifting, the power branch not affected by the shifting supports by means of its full power the tractive force at the torque output of the torque superposition device. In this case, the power branch affected by the shifting or the corresponding drive assembly (electric machine or internal-combustion engine) can be used for the “external synchronization” of the shifting.
In other words, in contrast to conventional parallel hybrid concepts, this results in the advantage that the electric machine and the internal-combustion engine in the parallel hybrid operation do not always have to rotate at a certain, fixedly predefined rotational speed ratio but that at least one of the two driving machines can be “shifted into another gear”, which is advantageous not only with respect to consumption aspects but provides the drive with better dynamics.
According to a further development of the invention, the second transmission device assigned to the internal-combustion engine has at least two and no more than four, preferably exactly three alternatively selectable transmission positions.
In this case, the at least two transmission positions of the second transmission device are to be designed such that one transmission position is constructed as a driving gear suitable for maximum speed, and a further transmission position is constructed as an efficiency gear. By means of the driving gear suitable for maximum speed, the maximum speed of the vehicle can be driven. In contrast, the efficiency gear is designed to be longer than the driving gear suitable for the maximum speed and thereby permits an efficiency-optimized operation of the internal-combustion engine in the medium or higher speed range, for example, on an express way or on the turnpike (autobahn).
In the case of a limitation to two transmission positions of the second transmission device designed in this manner, the first transmission device of the electric motor and also the power of the electric motor are to be designed such that the driving operation of the vehicle during the start of the drive and in the lower speed range, i.e. in the city operation, will be represented essentially by the electric motor. Here, the internal-combustion engine, as a rule and if required, carries out an assisting function by providing a base load.
A hybrid drive according to the invention that is limited to two transmission positions of the second transmission device is suitable for highly electrified drives with a fairly high electric power; i.e. the peak power of the electric motor should not be less than 30%, preferably at least 50%, even better at 80% and ideally at least 100% of the nominal power of the internal-combustion engine. The continuous power of the electric motor may in each case be approximately at a ratio of one-half below the peak power customary for electric motors.
The more balanced the power situation between the peak power of the electric motor and the nominal power of the internal-combustion engine, the better the electric motor will be able to cover the dynamic load demand, particularly during acceleration operations. Especially in the case of gear changes, the drive assemblies can also assist one another, which results in very comfortable shifting operations without or with only a slight breakdown of the tractive force during the shifting.
It is especially advantageous for the power ratio between the continuous power of the electric motor and the nominal power of the internal-combustion engine to be approximately balanced or to exceed the nominal power of the internal-combustion engine; i.e. the continuous power of the electric motor is approximately 80% of the nominal power of the internal-combustion engine or more. In this case, the electric motor can also generate the propulsion power in those driving situations in which the driving gear of the internal-combustion engine cannot yet be operated or can be operated only at low power—thus, in the case of a torque superposition device constructed according to the invention with, for example, only two internal-combustion engine gears designed as indicated above, when starting to drive and in the lowest speed range up to approximately 30 km/h−, and therefore can constitute the driving function purely or predominantly by means of the electric motor, without limiting the driving power.
If the peak power of the electric motor were lower than the above-specified minimum power demand, the electric motor could no longer constitute an adequate driving function but, if required, could still “assist” the driving mode of the internal-combustion engine. As a result, the reduction according to the invention of the currently customary six or more internal-combustion engine gears to a few gears with the above-mentioned advantages of the invention could no longer be represented.
As a result of the small number of transmission positions of the second transmission device, the component-side expenditures are low while particularly the space requirements are also low.
According to a further development of the invention, it may, however, also be advantageous to construct a further transmission position of the second transmission device as a starting gear. In this case, a starting element, such as a starting clutch, could be integrated in the hybrid drive. The starting gear can simultaneously be used as a hill gear in the lower speed range and ensure that a driving operation by the internal-combustion engine is feasible also in situations in which the energy content of the electric storage system of the vehicle cannot provide sufficient energy for a driving operation.
Particularly in the case of comparatively low electric motor powers according to the above-indicated preferred ranges, it may make sense with respect to dynamics to provide a still further, fourth, transmission position of the second transmission device, which, on the transmission side, is arranged as a further driving gear but is not suitable for maximum speeds, between the starting gear and the driving gear suitable for maximum speeds. This fourth transmission position will be used particularly when an acceleration takes place that originates from the lower speed range.
Although the first transmission device may be implemented with a single non-shiftable transmission position or even in a direct manner (transmission ratio 1:1), it is particularly advantageous for also the first transmission device to be implemented with at least two alternatively selectable transmission positions.
The transmission position with the “longer” design should preferably be designed as a gear of the vehicle that is suitable for maximum speeds. This has the advantage that the transmission position with the shorter design can be used as a dynamics gear with an excellent torque provision that is typical of e-motors, whereas the transmission with the transmission position with the longer design makes it possible to leave the e-motor in a coupled state also in the high speed range to the maximum speed range of the vehicle and to thereby utilize it for the braking energy recovery as well as for boosting.
According to a further development of the invention, the hybrid drive has precisely a single electric machine provided for generating the propulsion of the vehicle, which permits a comparatively simple, compact and cost-effective construction. This naturally also comprises those solutions in which an additional electric machine is integrated in the overall system, this electric machine then not being used for generating propulsion.
According to an advantageous further development of the invention, the torque output of the torque superposition device is constructed as an output shaft with an output gear position, the output gear position preferably being connected directly with the differential of the driven vehicle axle of the motor vehicle.
For selecting the individual transmission positions of the transmission devices, form-locking or frictionally engaged “shifting elements” can be provided. Gearshift sleeves, for example, may be provided in the case of spur gearwheel positions. In order to improve the shifting comfort, one synchronizing device respectively may be assigned to one of the form-locking shifting elements or to all form-locking shifting elements. Since an “external” pre-synchronization by means of the internal-combustion engine or by the electric machine is already conceivable, simpler cost-effective shifting elements may also be used, such as sliding sleeves with jaw clutches without synchronizing units, as known, for example, from motorcycle transmissions. An “external synchronization” can be implemented by a corresponding rotational speed control of the internal-combustion engine or of the electric machine.
When planetary transmissions are used as transmission devices, particularly the use of friction clutches or friction brakes may be considered. However, in principle, frictionally engaged shifting elements could also be used in the case of shiftable spur gearwheel positions.
An uncoupling of the electric machine or of the internal-combustion engine from the torque superposition device can very easily take place in that the corresponding shifting element or elements is(are) shifted into a “neutral position”. A “neutral position” is a position in which no torque transmission is possible from the electric machine or from the internal-combustion engine to the torque superposition device. As a result, a purely electric traveling drive is permitted as well as an uncoupling of the internal-combustion engine for reducing drag torques or a drive purely by the internal-combustion engine, in which case, the electric machine is uncoupled.
In summary, particularly the following advantages are achieved by means of the invention:                The independent shiftability of the two power branches permits a mutual tractive power assist. This increases the comfort compared to conventional hybrid concepts, in which an engaged transmission gear is always “impressed” on both power branches.        The hybrid concept according to the invention permits a free scalability of the drive assemblies and is therefore suitable not only for conventional hybrid concepts but especially also for so-called plug-in hybrid concepts.        The independent selection of the operating points for the internal-combustion engine and the electric machine opens up high potentials for reducing the consumption and emission, for improving the drive acoustics as well as for increasing the driving performances.        By means of the “summation function” of the torque superposition device, the internal-combustion engine and the electric machine can be optimally supplemented in the operation. This results in savings and efficiency potentials, which, in turn, permits “making” the two drive assemblies “less demanding”. “Making less demanding” means that, in contrast to conventional hybrid concepts, the electric machine no longer has to meet the highest efficiency demands over the entire rotational speed range.        By means of the hybrid drive according to the invention, a mixed operation, i.e. both drive assemblies are engaged and, as a function of the driving situation, supply positive (driving) torque or negative torque (applies only to the e-motor in the operation as a generator), or also a purely electric operation in the sense of an e-driving function, can be selectively exhibited in the driving operation.        
In the following, the invention will be explained in detail by means of the drawings.